This invention relates to a variable capacitance pressure transducer capable of handling higher pressures than would normally be tolerable with a single diaphragm type construction for the materials used. It also relates to a method for manufacturing such a transducer.
Pressure transducers such as those shown in U.S. Pat. No. 4,257,274, Shimada et al, issued on Mar. 24, 1981, have been developed to take advantage of the ease of manufacture and the favorable characteristics of silicon as a diaphragm material in combination with plates of borosilicate glass anodically bonded to its faces. The etched recesses in the diaphragm faces form with the glass plates opposing cavities which are very accurately dimensioned by the etching process so as to provide a pressure transducer structure which is stable with temperature changes. This type of pressure transducer also has benefits based on the considerable simplicity of its construction which makes it very easy to mass produce and which makes possible a miniaturization without sacrifice of the necessary characteristics of a good transducer.
A method for producing pressure transducers of the type described in the above mentioned patent is shown in U.S. Pat. No. 4,261,086 issued to Giachino on Apr. 14, 1981. This method calls for the manufacture of a number of transducers from a single wafer of silicon which is sliced into individual transducers as a last step in the manufacturing process with the resulting convenience for using batch processing techniques.
Pressure transducers of the type described in the above patents all utilize a process called electrostatic bonding, sometimes referred to as anodic bonding, to attach two pyrex glass sensing electrode support plates to opposite faces of a silicon disc into which are etched circular recesses. The diameter of the recesses defines the diameter of the diaphragm area while the depth of the recesses defines the capacitor plate spacing. The deflection of the diaphragm when a pressure is applied to one side is sensed by a capacitance increase on one side of the diaphragm and a capacitance decrease on the opposite side. The pressure range which can be measured by the device is determined by the deflections from unit of pressure on the diaphragm which, in clamped diaphragm theory, is proportional to the diameter to the 4th power divided by the thickness to the 3rd power. EQU d.varies.pD.sup.4 /t.sup.3
where
D=diaphragm diamter
t=thickness
d=center point deflection
p=pressure difference
In order to change the range for the pressure transducers produced in accordance with the above technique, changes are made by changing the diaphragm diameter "D" and the diaphragm thickness "t". For instance, if the diameter is halved, the range is increased by 16 times, and if the thickness is doubled, the range is increased by 8 times. Theoretically, assuming a maximum deflection for the diaphragm, any range can be obtained by an appropriate choice of thickness and diameter. In practice, however, this is not the case since there are limitations in the value for thickness and diameter which can be practically achieved for a diaphragm. Thus, there is an upper pressure limit in the practical sense for a single diaphragm as established by these values.
For a fixed diameter the range can be increased by increasing the thickness of the diphragm. As the range increased, the forces acting on the cavity increase in proportion to the pressure times the cavity area. A pressure will be reached where these forces exceed the material strength even though a diaphragm thickness can be established for the proper deflection. In order to operate, the diameter of the cavity must be reduced instead of the diaphragm thickness being increased. The choice of dimensions will therefore be made first for strength of the device by specifying a diameter small enough to contain the pressure being measured. Second, a thickness of diaphragm is chosen to give the desired diaphragm deflection at the pressure being measured.
There is also a limitation in how small the diameter for the diaphragm can be made because as the diameter is decreased the capacitance of the unit is decreased as the square of the diameter. This becomes of significance when one considers that the electrical connections to the transducer always have some level of stray capacitance associated with them. Thus, as the diameter decreases and the device capacitance drops, the point is reached where the stray capacitance is too large compared with the device capacitance to make a linear measurement. A second problem arises from the use of small diameter diaphragms in that the measuring signal, which is typically an AC current, becomes too small because the impedance of the device is too high and the accuracy degrades because of signal to noise or temperature coefficient limitations.
It is an object of this invention to provide a transducer structure and a method of manufacture which can provide a unit which can exceed the upper pressure limits of the devices of the prior art.